Such a monitoring system has been described in my prior U.S. Pat. Nos. 3,747,010, 3,747,011, 3,747,012, 3,919,661, 3,932,774, 3,932,803 and 3,935,542. As particularly disclosed in U.S. Pat. No. 3,932,803, the contactless motion detector may comprise an oscillator and an associated trigger amplifier whose energizing circuit includes a storage capacitor which is charged via a two-conductor supply network from a source of pulsating direct current by way of a constant-current device in parallel with the anode/cathode path of an output thyristor. In one embodiment of the system described and claimed in the latter patent, the output thyristor and the constant-current device lie in series with an ancillary thyristor which shunts the storage capacitor and is separated therefrom by a decoupling diode. When the output thyristor is triggered by a signal from the oscillator, the ancillary thyristor is fired through a Zener diode inserted between its anode and its gate; thus, the two series-connected thyristors conduct simultaneously in series with a load such as a relay whose operated (or unoperated) state indicates an abnormal condition, e.g. the fact that the oscillator has detected the approach of a metallic element.
Through the system just described operates generally satisfactorily, a drawback resides in the fact that the load is energized through a resistance representing the sum of the resistances of the two series-connected thyristors in their conductive state.